I posted this as well on .net so I apologize to anyone reading it twice. Moral of the story: DON'T POLISH YOUR VALVECOVER.
I'm not one to normally do anything to my car that doesn't improve its performance to some degree. However, I had recently found/polished a gigantic piston from an old diesel compressor (pic below) and I had the idea to polish my valve cover.
Firstly, valve cover removal - very easy. If it is your first time removing wires/vac lines/etc just label everything with post-its or whatever. I was very impressed how clean everything was underneath.
Secondly, remove all the rubber grommets and metal baffling on the underside of the piece. Scrape off the silicone sealmaker.
Sandblast, this was easy and cheap for me because we have a pretty amazing sandblasting booth at work. Time about 1.5 hours. Then get out your palmsander/airsander/rotary tool and start sanding. Take this opportunity to sand off the casting lines around the edges. The more time you spend getting in all the tiny curves and hard to reach areas by hand with the really rough paper (80 grit) the better your finished product will look. I spent about 2 hours per grit level. Rinse and repeat for 120, 180, 220 and 400 grit. I realize now that 2 hours per grit isn't enough. Not even close to enough.
At this point what you should be doing is going to a 400 wetgrit and then a 800 wetgrit, followed by an Aluminum polish like mother's AL polish or a Jeweler's roush bar. But this is not how it went. I was so happy with how the piston turned out that when I polished a sample area on the cover my heart fell. Piston = machined, Cover = cast. You simply are not going to get that mirror finish that you can shave in with a metal that hasn't been machined. Sure it was shiney and reflected light well, but images were badly distorted and you could see thickness imperfections on the surface. Also, anyplace you don't go over and over with will look deeply pitted under close inspection (ie. hard to reach small creveses).
Beaten, annoyed, and without working miata, I finally decided to powdercoat the valve cover. Car is white, so white it is. The powdercoating was pretty decently priced, and afterwards I got artistic and got some BBQ black paint and painted in the lettering. Have to admit, it looks really good. Its not what I wanted orginally but it turned out for the better. What does everybody think?
After 180 grit sandpaper
After 220 grit sandpaper
Really bad picture of some of the tiny little areas that you HAVE to sand
Sample area polished. You can see the poor reflection here
Finished product. I painted the lettering in 5 very light coats, careful to wipedown and over-edging